When uploading SVGs into the media library, Prismic is changing the stated dimensions of SVGs.
Firefox (worst case): Uploads them as 0x0 objects - will not display the SVG when it's on the page.
Chrome (medium case): Uploads the image but changes the dimensions to ~150px wide. Works on page, but is teeny tiny if you don't / can't place a width:100% on the SVG object.
Safari (best case): Uploads the SVG without altering it's sizing.
Frustratingly, downloading the uploaded files doesn't have any difference - the files are exactly the same. But you can see Primisic reporting the sizes here, and it does affect the way the image is rendered when placed onto a page.